Use of multi-beam antennas is common in applications such as cellular communications, where benefits including increased range and improved signal reception may be achieved. For example, rather than providing coverage of a 120 degree azimuth sector at a cell site with an antenna providing a single 120 degree beam, sector coverage may be provided by a multi-beam antenna having a higher gain radiation pattern including four 30 degree beams.
It will be appreciated that efficiency and effectiveness of coverage by the multi-beam antenna will depend on factors such as signal loss or dissipation in a feed network used to provide the four beam pattern and sidelobe characteristics which may permit operation to be degraded in the presence of interference or noise effects.
Efforts have been made to design multi-beam feeds capable of providing lossless operation, well known Butler networks being one example. For present purposes, "lossless", in the context of a multi-beam feed network, is defined as a general absence of resistive elements at a pattern of locations in the feed network, although a small number (e.g., one resistive termination) of such elements may be present. Thus, lossless feed networks are not absolutely lossless, but are much less lossy than a feed network including resistive elements in a series of parallel paths or at many directional couplers in a coupling matrix, for example. Lossless feed configurations are discussed in Hansen, R. C., Microwave Scanning Antennas, Vol. III Array Systems, Academic Press, 1966, at pages 258-263.
Relevant constraints in achieving both required operating characteristics and lossless operation of multi-beam feed networks is discussed, for example, in a paper entitled "Optimum Low Sidelobe High Crossover Multiple Beam Antennas", by E. C. DuFort, appearing in IEEE Transactions on Antennas and Propagation, Vol. AP-33, No. 9, September 1985, at pages 946-954. This paper makes reference to prior development of certain specific forms of lossless array feeds and points out that beams must be mutually orthogonal in order to be derived from a lossless feed network. The paper states the conclusion that prior efforts toward development of similar types of lossless networks, with higher beam crossover points and lower sidelobes, was generally not successful in providing feeds suitable for applications of interest.
Thus, while the advantages of low sidelobe lossless feed networks have been recognized, practical feed networks of this type suitable for particular applications have not been available in the prior art. In this context, it is relevant to observe that for many current cellular applications achievement of high crossover radiation beam characteristics is somewhat less critical than in other applications. Lossless operation and low sidelobe characteristics are important in cellular applications.
It is, therefore, an objective of the present invention to provide new and improved lossless feed networks suitable for cellular and other applications. More particularly, objects of the invention are to provide feed networks having one or more of the following characteristics and capabilities:
lossless (minimized resistive loss) operation; PA1 low sidelobe radiation pattern; PA1 multi-beam operation with orthogonal excitation outputs; PA1 aperture ports fed by a smaller number of beam ports; PA1 a larger number of aperture ports than radiated beams; PA1 economical, reliable design; and PA1 improved cellular performance. PA1 beam port A input, aperture port outputs: I=0.7; II=1; III=1; IV=1; V=0.7; PA1 beam port B input, aperture port outputs: I=0.7; II=-1; III=1; IV=-1; V=0.7; PA1 beam port C input, aperture port outputs: I=0.7; II=j; III=-1; IV=-j; V=0.7; PA1 beam port D input, aperture port outputs: I=0.7;, II=-j; III=-1; IV=j; V=0.7.